A lathe is a rotational metal machining tool that is commonly used in manufacturing and prototyping articles having a degree of cylindrical symmetry prior to other processes. The lathe includes a spindle and a cutting tool. An article to be machined, referred to as a workpiece, is securely mounted in the spindle. The spindle then spins, rotating the workpiece. The cutting tool is then manipulated to impinge upon the workpiece whereby it makes circular cuts into the workpiece. Mounting and manipulation of the cutting tool is used to control a cylindrical cut geometry that defines resultant geometrical modifications of the article by the lathe.
The subject of this invention relates to the spindle and an adaptive apparatus for mounting the workpiece in the spindle. Spindles come with work-holding apparatuses of various types including jaw-type chucks and collets. A collet is typically used for a small to medium sized workpiece. Collets come in various types including draw-in collets, stationary collets, and expanding collets. This invention relates to the draw-in and stationary collets. The following discussion concerning FIGS. 9A and 9B will be helpful in understanding the invention.
For future descriptions the terms forward and rearward are used. Forward always refers to an axial direction (along the axis of spindle rotation) out of the spindle. Rearward always means the opposite of forward. To illustrate this convention a workpiece is inserted into a collet in the rearward direction and removed in the forward direction.
FIG. 9A illustrates a cross-sectional view of a spindle 100 including an axially stationary spindle sleeve 102 and draw-in collet 104. After a workpiece is placed in collet 104 a draw tube 106 attached to collet 104 pulls collet 104 rearward (to the left in the figure, into the spindle). As this happens, a flared zone of sleeve 102 bears radially inwardly upon a similarly flared zone of collet 104 in order to close collet 104 upon the workpiece. The sleeve 102 remains axially fixed while the collet 104 is pulled axially in a rearward direction. The process of closing a draw-in collet 104 has an axial positional component that decreases accuracy in axial location.
FIG. 9B illustrates a cross sectional view of a spindle 200 including an axially movable spindle sleeve 202 and a stationary collet 204. Surrounding the axially movable spindle sleeve 202 is a fixed portion 206 of spindle 200 including cap nut 208. After a workpiece is placed in collet 204, the spindle sleeve 202 moves forward. As this happens, a flared zone of sleeve 202 bears radially inwardly upon a similarly flared portion of collet 204 in order to close collet 204 to provide clamping force upon the workpiece. During this process collet 204 remains stationary, being held in place by cap nut 208 which is part of the fixed portion 204 of spindle 200. In contrast to the draw-in collet, the stationary collet closure does not have an axial positional component. Because of this, stationary collets are preferable for some manufacturing operations and for machining that requires high axial precision.
Lathes are generally used in fabricating articles having a wide range of diametrical sizes. The size of the spindle generally correlates with the maximal diameter of a workpiece. This invention concerns a need to fabricate relatively small articles such as small mechanism components and dental implants. Ideally smaller lathes are used for smaller parts. However there are economical and delivery rate reasons why a shop would like to be able to use larger lathes for small parts. For example, if a shop has a number of currently under-utilized larger lathes, an order for smaller articles can be accomplished more quickly if all the idle lathes can be used.
There is also a motivation to be able to machine a small workpiece as close as possible to an axial location at which the workpiece is supported by the collet. When conventional stationary collets are used the distance between axial collet support and machining may become too large such that the workpiece is not adequately supported. The spindle cap design generally used for stationary collets is inherently bulky in diameter and also necessitates an axial extension of the work zone away from the flared clamping area. These aspects degrade workpiece clamping force at the point of machining and necessitate the extension of cutting tools, which reduces strength, rigidity, and accuracy. What is needed is a way of machining such a small diameter workpiece very close to the support zone.